Generally, a semiconductor light-emitting device has a light-emitting diode (LED) and a laser diode (LD) generating light when a forward current flows. Particularly, the LED and LD have a common p-n junction, and when a current is applied to the light-emitting device, the current is converted to photons, and light is emitted from the device.
The light emitted from the LED and LD has various wavelengths from a long wavelength to a short wavelength depending on the semiconductor material(s). Above all, LEDs made from wide band-gap semiconductors allow red, green and blue colors in visible bands and have been applied widely in industries such as displays for electronic devices, traffic lights, and various light sources for display devices. Due to the development of white light in recent years, it will be widely used as the next generation light source for general lighting.
A Group III-V nitride-based semiconductor is generally produced hetro-epitaxially on the upper part of sapphire to obtain high quality semiconductor thin films, and sapphire, silicon carbide (SiC), silicon (Si) and the like have been used as an initial substrate.
Among them, the sapphire substrate has a significantly different lattice constant and thermal expansion coefficient compared to those of the Group III-V nitride-based semiconductor, and thus it has been difficult to laminate a multi-layered light-emitting structure thin film including the Group III-V nitride-based semiconductor. Further, since the sapphire substrate has poor thermal conductivity, a large current cannot be applied to LEDs. Since the sapphire substrate itself is an electrical insulator and thereby is difficult to respond to static electricity flowed in from outside, it has a high possibility to cause failure due to the static electricity. Such drawbacks not only reduce reliability of devices but also cause a lot of constraints in packaging processes.
Further, the sapphire substrate, which is an insulator, has a MESA structure, in which both an n-type ohmic contact electrode and a p-type ohmic contact electrode are formed in the same growth direction as that of a multi-layered light-emitting structure including the Group III-V nitride-based semiconductor. Since an LED chip area should be higher than a certain size, there is a limit to reducing the LED chip area, restricting the improvement of LED chip production.
In addition to these disadvantages of the MESA-structured LEDs grown on the upper part of the sapphire initial substrate as an initial substrate, it is difficult to release a great amount of heat outward generated inevitably during the operation of the light-emitting device since the sapphire substrate has poor thermal conductivity.
Due to these reasons, there is a limitation in applying the MESA structure, to which the sapphire substrate is attached, to light-emitting devices used for a large area and a large capacity (that is, a large current) such as the light for large displays and general lighting. When a high current is applied to a light-emitting device for a long period of time, the internal temperature of a light-emitting active layer is gradually increased, largely due to the generated heat, and thereby an LED light-emitting efficiency is gradually decreased.
A silicon carbide (SiC) substrate, unlike the sapphire substrate, not only has good thermal and electric conductivities but also allows a multi-layered light-emitting structure thin film to be laminated and grown since it has a similar lattice constant and thermal expansion coefficient (TEC), which are important factors in the semiconductor single crystal thin film growth, as that of the Group III-V nitride-based semiconductor. Further, it allows the manufacturing of various types of vertically-structuredvertically-structuredvertically-structuredvertically-structured light-emitting devices.
However, because producing a high quality SiC substrate is not easy, it is more expensive than producing other single crystal substrates, making it difficult for mass production.
Therefore, it is most desirable to provide a high-performance light-emitting device by using a multi-layered light-emitting structure including the Group III-V nitride-based semiconductor laminated and grown on a sapphire substrate in view of the technology, economy and performance.
As described above, much effort has been made to produce a high-performance vertically-structured LED by growing a high quality multi-layered light-emitting structure thin film including the Group III-V nitride-based semiconductor on the upper part of sapphire, SiC or Si, etc. of the initial substrate, lifting off the multi-layered light-emitting structure thin film including the Group III-V nitride-based semiconductor from the initial substrate and using the result, in order to resolve the problems associated with the MESA-structured LEDs produced by using a thin film which is a multi-layered light-emitting structure including A Group III-V nitride-based semiconductor laminated/grown on the upper part of a sapphire substrate which is an initial substrate.
FIG. 1 is a sectional view illustrating a process for separating a sapphire substrate by employing a conventional laser lift off (LLO) process.
As shown in FIG. 1, when a laser beam, which is a strong energy source, is irradiated to the backside of a transparent sapphire substrate 110, the laser beam is absorbed strongly at an interface 120, and the temperature of 900° C. or higher is thereby generated momentarily and causes thermochemical dissociation of gallium nitride (GaN), InCaN at the interface, and further separates the sapphire substrate 110 from a nitride-based semiconductor thin film 130.
However, it has been reported in many documents that in the laser lift-off process of the multi-layered light-emitting structure thin film including the Group III-V nitride-based semiconductor, the semiconductor single crystal thin film is damaged and broken after being separated from the sapphire substrate due to a mechanical stress generated between the thick sapphire substrate and the Group III-V nitride-based semiconductor thin film because of the difference in the lattice constant and thermal expansion coefficient.
When the multi-layered light-emitting structure thin film is damaged and broken, it causes a large leaky current, reduces the chip yield of light-emitting devices and reduces the overall performance of the light-emitting devices. Therefore, studies are currently under way for manufacturing a high-performance vertically-structured LED by using the lift-off process of the sapphire substrate that can minimize damage to the multi-layered light-emitting structure thin film, and the separated multi-layered light-emitting structure thin film.
As a result, various methods have been suggested to minimize the damage and breaking of the multi-layered light-emitting structure thin film including the Group III-V nitride-based semiconductor when the sapphire substrate is separated by the LLO process.
FIG. 2 is sectional views illustrating a process for forming a conductive support closely attached in the growth direction by employing a wafer bonding and electro plating process prior to the laser lift off process according to a conventional technology to prevent damage and breaking of a semiconductor multi-layered light-emitting structure thin film.
Referring to (a) in FIG. 2, a conductive support 250, which is strongly attached and is structurally stable, is formed by using wafer bonding on the upper part of a bonding layer 240a before lifting off multi-layered light-emitting structure thin films 220, 230 including a Group III-V nitride-based semiconductor from a sapphire substrate 210 by irradiating the backside of the transparent sapphire substrate with a laser beam. Referring to (b) in FIG. 2, a conductive support 250, which is strongly attached and is structurally stable, is formed on the upper part of a seed layer 240b by using an electro plating process before lifting off multi-layered light-emitting structure thin films 220, 230 from a sapphire substrate 210.
FIG. 3 is a sectional view illustrating vertically-structured Group III-V nitride-based semiconductor light-emitting devices manufactured by introducing the conductive support, which is strongly attached and is structurally stable, according to the conventional technology used in the process of FIG. 2.
The figure indicated by (a) in FIG. 3 is a sectional view illustrating a semiconductor light-emitting device manufactured by using the method for manufacturing the conductive support indicated by (a) in FIG. 2. Referring to (a) in FIG. 3 illustrating a vertically-structured LED section bonded with a wafer bonding, it is successively constituted with a support 310, which is a thermal and electrical conductor, a bonding layer 320a, a multi-layered metal layer including a p-type ohmic contact electrode 330, a p-type semiconductor cladding layer 340, a light-emitting active layer 350, an n-type semiconductor cladding layer 360, and an n-type ohmic contact electrode 370. A semiconductor wafer such as silicon (Si), germanium (Ge), silicon-germanium (SiGe), gallium arsenide (GaAs) and the like having an excellent thermal and electrical conductivity is preferably used as the support 310.
However, the support 310, used for the vertically-structured light-emitting device (LED) as shown in (a) of FIG. 3, causes significant wafer warpage and fine micro-cracks inside the multi-layered light-emitting structure thin film when Si or another conductive support is bonded by wafer bonding because it has a significant difference in thermal expansion coefficient (TEC) against the sapphire substrate on which the multi-layered light-emitting structure thin films 340-360 are grown/laminated. Such problems further cause processing difficulties and lower the performance of an LED manufactured therefrom and the product yield.
The figure indicated by (b) in FIG. 3 is a sectional view illustrating a semiconductor light-emitting device manufactured by using the method for manufacturing the conductive support indicated by (b) in FIG. 2. Referring to (b) in FIG. 3 illustrating the sectional view of the vertically-structured LED formed through electro plating, the vertically-structured light-emitting device (LED) formed through an LLO and electro plating process has the same structure of (a) in FIG. 3, except having a seed layer 320b, instead of the bonding layer 320a. The electrically conductive support 310, which is a thick metal film formed through electro plating, is preferably formed with a single metal such as Cu, Ni, W, Au, Mo and the like or an alloy composed thereof.
The LED support 310 having the structure described above as shown in (b) of FIG. 3 has a significantly higher thermal expansion coefficient and flexibility than the sapphire substrate due to the metal or alloy thick film formed through electro plating, thereby causing curling, wafer warpage, breaking, etc. during a “chip manufacturing process” such as mechanical sawing or laser scribing, etc.
Therefore, it is highly demanded to develop highly efficient heat sink supports suitable for manufacturing the high performance vertically-structured light-emitting devices so as to resolve the problems of wafer warpage, breaking, micro-crack, limitations in post-processing including annealing and chip processing, low product yield, etc. while manufacturing the vertically-structured Group III-V nitride-based semiconductor light-emitting device using the LLO process.